Knowledge Base

PRB: Winsock Control Generates Error 10048 - Address in Use

Article ID: 173619

Article Last Modified on 4/28/2000

APPLIES TO

This article was previously published under Q173619

SYMPTOMS

The Winsock control will get an error after a Connect method is called if the same local port has been used within the past four minutes.

CAUSE

The Windows socket control does not have the ability to set the socket option SO_REUSEADDR. This option allows the socket to be bound to an address that is already in use.

When you close the handle to a socket, some additional negotiation goes on between the client and the server. The socket will wait for up to two times the maximum time that windows would wait to receive an acknowledgement from the other end of the socket that closed the port. By default, this option is set to two minutes. Therefore, Windows may wait up to four minutes before the port is actually released.

This makes that specific port unavailable until it is actually released.

RESOLUTION

The only workaround is to not use a specific local port. If you set the LocalPort property to Zero, Winsock will pick a random local port for you and use it until a Close method is called on the WinSock Control.

STATUS

This behavior is by design.

MORE INFORMATION

Setting the LocalPort property of the socket to Zero, will cause Windows to pick a random port.

To do this, place the following code just before the line that does the connect: 
   Winsock1.LocalPort = 0
   Winsock1.Connect

Steps to Reproduce Behavior

The following sample demonstrates how to reproduce the issue and how to work around it. In the code "myserver" indicates any Windows 2000 or Windows NT Server running Simple TCP/IP Services, which includes an Echo server on Port 7.
  1. Start a new project in Visual Basic. Form1 is created by default.
  2. Add the Windows Socket Control to Form1.
  3. Add the following code to the form:
          Option Explicit

      Private Sub Form_Load()
        Dim res As Long
        Winsock1.LocalPort = 2500
        Winsock1.RemoteHost = "myserver"
        Winsock1.RemotePort = 7
        Winsock1.Connect
      End Sub

      Private Sub Winsock1_Error(ByVal Number As Integer, _
        Description As String, ByVal Scode As Long, _
        ByVal Source As String, ByVal HelpFile As String, _
        ByVal HelpContext As Long, CancelDisplay As Boolean)

        MsgBox Description, vbOKOnly, "Winsock Error: " & Number
        CancelDisplay = True
      End Sub
  4. Run the program.
  5. Exit the program.
  6. Run the program again. Note that this causes a dialog box to appear with the error message:
    Address in use.
    To correct the code, change the line: 
       Winsock1.LocalPort = 2500
    to: 
       Winsock1.LocalPort = 0
    and perform steps 4 through 6 several times. You should not get any errors.

Keywords: kberrmsg kbwinsock kbapi kbnetwork kbprb kbctrl KB173619


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